The invention relates to a method for removing molten glass from flow channels for the transport of production glass, these channels being installed between a melting furnace and an extraction point for the production glass, whereby the flow channel has a glass-resistant inner lining, the exterior of which is surrounded by mineral thermal insulation material and whereby a drainage appliance for bottom glass is installed upstream of the extraction point for the production glass.
In order to be able to evaluate the state-of-the-art and the invention it is advisable to consider, on the one hand, details of the constructional elements of heated flow channels that are also referred to as feeders or forehearths and, on the other hand, the elements of the electrical current paths and the heat thereby produced in such flow channels.
Both the mineral internal surfaces of the flow channels and the metal external surfaces of electrodes are susceptible in varying degrees to attack by normal glass melts, whereby the reaction products are heavier than the molten glass and collect on the bottom of the flow channel in the form of contaminated bottom glass. The reaction products of the glass with metal electrodes that are immersed directly in the molten glass are particularly damaging. Other contaminants such as stones and knots are also deposited in the bottom glass.
The molten glass above the bottom glass is intended for the manufacture of products and therefore termed production glass. This production glass is removed at an extraction point, either continuously, e.g., for the production of flat glass, or in portions, e.g., as gobs for the manufacture of bottles and drinking vessels.
If the bottom glass that is separated from the production glass by a phase boundary is not removed, either continuously or intermittently, through a drainage opening upstream of the extraction point for the production glass, then the production glass becomes contaminated by the bottom glass and cannot be used. Among other things, this leads to impairment of the transparency, for example as a result of discolouration and/or the formation of cords, known in the industry as “cat scratches.”
Another problem area lies in the type and shape of the mineral materials used for the flow channel and their geometric location relative to the electrodes. If, as is known, the inner lining of the flow channel comprises of a fusion cast material from the group of AZS or ZAC materials, such as an Al2O3—ZrO2—SiO2, also known as ternary systems, then the electrical conductivity is approximately 20 times higher than that of normal mineral materials, such as those used for the thermal insulation of the flow channels. As a result, the electrical current paths tend to run through such materials, whereby the shape of the materials and the spatial coordinates of these current paths within these materials and the passage and course of the current paths from the individual electrodes, to and in the glass melt, must also be determined. However, there is also interaction between these current paths and the spatial coordinates of the temperatures to consider.
This results in certain relationships between the relative values of the currents that flow through the mineral materials and the molten glass, and the resulting localised heating effect. The electrical conductivity, or the specific electrical resistance, for both the glass and the mineral materials are extremely temperature dependent over several orders of magnitude within a temperature range between 700 and 1700° C. The results are therefore based on numerous, intensive and expensive tests, undertaken until the best possible solution is found. The rate of temperature change in the area around the drain opening also plays a role in this respect.